WDI Fundamentals

WDI Fundamentals Unit 8

Expressions & Variables Cheat Sheet

Here are some notes on what's been covered in this chapter. Feel free to copy this information and extend it to make your own cheat sheet.


  • A way to plan out your program before coding it. It is a detailed, step-by-step description of what a computer must do, expressed in plain English rather than in a programming language.


  • An expression is a statement composed of values/data and operators.
  • Some common data types are numbers, strings, and Booleans.
  • An operator takes in a number of inputs but outputs/evaluates to a single value.
  • To determine how an expression is evaluated, look at what each operator's inputs are and, if necessary, generate an expression tree to illustrate the expression's structure.


  • The purpose of variables is to store and reuse the values created from a computation.
  • A variable is assigned a value using the = operator. First, the expression to the right of the = is evaluated. Then, this value is assigned to the variable to the left of the =. Finally, the = operator evaluates to the value that has just been assigned.
  • To use the value that a variable is storing, simply include that variable in an expression. An expression containing variables will evaluate just like one without variables, except that the variables will themselves be evaluated as part of the expression. As before, it is possible to draw an expression tree to illustrate the expression's structure.
  • When a variable is redefined, it retains no knowledge of any prior values it may have held.
  • A variable may be redefined "in place" using an expression like x = x + 1 (or its shorthand, x += 1).
  • An expression like x = y only means that the value that y had been holding is now also held in x. It does not imply any lasting relationship between x and y.

Special Cases

  • When a variable is created but is not assigned a value, it will be evaluated as undefined.
  • Any type of value, including null, can be passed into a logical operator as an input; based on whether these inputs are either "truthy" or "falsey" and the type of operator you're dealing with, the operator will behave in different ways.

Comparison Operators

Operator Meaning True expressions
== Equality 10 == '10'
=== Strict equality (2 * 5) === 10
!= Inequality 9 != 10
!== Strict inequality '10' !== 10
> Greater than 20 > 10
>= Greater than or equal to '10' >= 10
< Less than 10 < 30
<= Less than or equal to '10'<= 10

Logical Operators

Logical operators work on Boolean values to produce Boolean results.

AND operator &&

Condition 1 Condition 2 Result
true true true
true false false
false true false
false false false

OR operator ||

Condition 1 Condition 2 Result
true true true
true false true
false true true
false false false

NOT operator !

Condition Result
true false
false true

We can use parentheses to change the order of operations for logical operators, just like we do in mathematics.

Feeling confident? Let's take what we've learned and apply it to a project.